Viral load of equine herpesviruses 2 and 5 in nasal swabs of actively racing Standardbred trotters: Temporal relationship of shedding to clinical findings and poor performance
Introduction
The closely related equine gamma herpesviruses 2 and 5 (EHV-2 and EHV-5) are poorly understood but under increasing scrutiny in the equine world. Herpesviruses are characterized by lifelong latent infection in the host, where the cells in which they remain latent vary dependent on type of herpesvirus (Roizmann et al., 1992). In humans, the gamma herpesvirus Epstein–Barr virus (EBV) is asymptomatic or causes very mild symptoms if the infection occurs in childhood, whereas infection later in life is associated with mononucleosis (Balfour et al., 2013a). Most reviews indicate that >90% of the adult population in the USA are persistently infected with EBV whereas the antibody prevalence in children is around 50% (Balfour et al., 2013b).
In horses, both gamma herpesviruses EHV-2 and EHV-5 infect individuals early in life (Murray et al., 1996, Bell et al., 2006, Wang et al., 2007). Therefore, they are commonly detected in all age groups in healthy equine populations (Kemeny and Pearson, 1970, Bell et al., 2006, Borchers et al., 2006, Torfason et al., 2008, Rushton et al., 2013) and were presumed to be largely non-pathogenic.
Recent reports have challenged these assumptions, with an increased frequency of reports linking presence of equine gamma herpesviruses with various clinical problems. For example, EHV-2 has been incriminated in mild respiratory disease (Dunowska et al., 2002, Wang et al., 2007), tracheal inflammation (Fortier et al., 2009) and keratoconjunctivitis (Kershaw et al., 2001), while EHV-5 has been linked to occurrence of equine multinodular pulmonary fibrosis (EMPF) (Williams et al., 2007). Earlier work also suggested that EHV-2 infections may be associated with poor athletic performance in race horses (Studdert, 1974). Previous studies have shown that younger horses are more likely to have a higher viral load of EHV-5 compared to adults (Marenzoni et al., 2010) and in an EMPF case viral load was higher in implicated organs, like the lung (Marenzoni et al., 2011). On the other hand, viral load of EHV-2 in a cohort study of foals followed over five months showed no relationship with occurrence of clinical signs (Brault et al., 2011). To date however, no studies have followed horses over time to evaluate the relationship between infection of equine gamma herpesvirus and poor performance. Thus, information about the pattern of viral load of gamma herpesviruses in the horse population is scanty, and the role of EHV-2 and EHV-5 in clinically detectable disease, in particular during periods of high viral shedding, is not known.
The aim of this work was to monitor qualitatively and quantitatively the excretion of EHV-2 and EHV-5 in the upper respiratory tract, as markers of viral load in elite racing Standardbred trotters to identify the dynamics of viral shedding within and between horses, and over seasons. Additionally results were used to assess whether variations in viral load could be associated with selected indices of poor performance or clinical respiratory disease. To accomplish this we also needed to develop and validate two single qPCR assays specific for EHV-2 respective EHV-5 targeting the DNA polymerase gene, as recently published methods were not available for us at the time (Rushton et al., 2013, Hue et al., 2014).
Section snippets
Description of the cohort
From August 2010 to August 2011 (13 months), a cohort of 66 actively racing Standardbred trotters was followed at monthly intervals. At the time of recruitment the horses were healthy and well performing with a mean age of 3 years, (range: 2–8 years, SD: 1.33) and kept at four different professional training yards, as previously described (Back et al., 2015). Throughout the study period, all horses trained and raced according to their normal schedule and the same veterinarian monitored their
Description of the cohort
Two of the 66 horses were excluded due to change of trainer and one horse was excluded upon request of the owner. From the remaining 63 horses a total of 663 NS samples were taken during the study period. Regarding occurrence of abnormal clinical signs at the time of sampling, 1% had fever (9/663 of which one horse had cough at the same occasion and another had nasal discharge together with fever), 1% cough (8/663) and 1% nasal discharge (6/663) (described in more detail in Back et al., 2015).
Validation of the qPCR assays
Discussion
In this longitudinal study, both EHV-2 and EHV-5 were commonly detected in nasal secretions from the horses, which is in agreement with previous studies worldwide (Wang et al., 2007, Pusterla et al., 2013, Hue et al., 2014). For EHV-5, 74% of the samples were positive on qPCR assay and 40% of the horses were uniformly tested positive throughout the study. Since a large group of horses was followed over a year, the prevalence of EHV-2 and EHV-5 presented in Table 1, Table 2 provides invaluable
Conclusions
It appears that both EHV-2 and EHV-5 are endemic in this population of racing Standardbred trotters and although there was a large variation in viral load, both within individual horses and between EHV-2 and EHV-5, and over seasons. However, neither the presence of EHV-2/-5, nor the timing of the peak of the viral load was associated with clinical respiratory signs, markers of systemic inflammation, pharyngeal or tracheal inflammation or poor performance. Since the peaks of viral load of EHV-2
Acknowledgements
The authors wish to acknowledge Dr. Nils Ronéus for scientific contribution and thank the trainers and owners without whom this research would not have been possible. This study was funded by Swedish-Norwegian Foundation for Equine Research project no H1247140, and the endoscopic equipment was provided by Scandivet, Sweden.
References (32)
- et al.
Temporal detection of equine herpesvirus infections of a cohort of mares and their foals
Vet. Microbiol.
(2006) - et al.
Prevalence of equine herpesvirus type 2 (EHV-2) DNA in ocular swabs and its cell tropism in equine conjunctiva
Vet. Microbiol.
(2006) - et al.
Genetic heterogeneity and variation in viral load during equid herpesvirus-2 infection of foals
Vet. Microbiol.
(2011) - et al.
Kinetics of Equid herpesvirus type 2 infections in a group of Thoroughbred foals
Vet. Microbiol.
(2011) - et al.
Long-lasting airway inflammation associated with equid herpesvirus-2 in experimentally challenged horses
Vet. J.
(2013) - et al.
Herpesviruses in respiratory liquids of horses: putative implication in airway inflammation and association with cytological features
Vet. Microbiol.
(2009) - et al.
Detection and quantitation of equid gammaherpesviruses (EHV-2, EHV-5) in nasal swabs using an accredited standardised quantitative PCR method
J. Virol. Methods
(2014) - et al.
Detection of equine herpesvirus type 2 (EHV-2) in horses with keratoconjunctivitis
Virus Res.
(2001) - et al.
Detection of equid herpesviruses 2 and 5 in a herd of 266 Lipizzaners in association with ocular findings
Vet. Microbiol.
(2013) - et al.
Study of equid herpesviruses 2 and 5 in Iceland with a type-specific polymerase chain reaction
Res. Vet. Sci.
(2008)
Detection of respiratory herpesviruses in foals and adult horses determined by nested multiplex PCR
Vet. Microbiol.
A longitudinal study of poor performance and subclinical respiratory viral activity in Standardbred trotters
Vet. Rec. Open
A prospective clinical study of Epstein–Barr virus and host interactions during acute infectious mononucleosis
J. Infect. Dis.
Behavioral, virologic, and immunologic factors associated with acquisition and severity of primary Epstein–Barr virus infection in university students
J. Infect. Dis.
Age-specific prevalence of Epstein–Barr virus infection among individuals aged 6–19 years in the United States and factors affecting its acquisition
J. Infect. Dis.
Viruses associated with outbreaks of equine respiratory disease in New Zealand
N. Z. Vet. J.
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These authors contributed equally to the study.